1. Field of the Invention
The invention herein relates to a process of preparing a novel polyimide foam having superior heat-resistance, flame retardant, homogeneous size and distribution of cells, and low density, wherein a polyimide precursor in a granular form is prepared by means of using heterocyclic amine as catalyst and then foaming.
2. Description of the Prior Art
In general, the polymer foam refers to a light substrate in which the cells within the polymer are well distributed with low density. The foam is widely used as basic material in adiabatic, cushion, shock absorbent, soundproof material, and packaging material due to its superior cushion, adiabatic, soundproof, vibration-proof properties. The general purpose cells such as polyurethane or polyolefin cells are widely used in automobile, ships, and transportation equipment due to their superior strength, restoring capacity, adiabatic property, preparatory and economic efficiency. However, for a use in aerospace, submarine, special ships and high-speed train, which require superior heat-resistance and flame retardant, a special heat-resistance foam with superior heat-resistance and flame retardancy properties is used therein. With an aromatic or heterocyclic structure, a typical heat-resistance foam includes polyimide with high heat-resistance. In particular, the polyimide foam is known to have a wide application due to its superior thermal stability and flame retardancy.
The preparing process of a polyimide-based foam includes carrying out a foaming by means of heating a polyimide in an oligomer condition in an oven or microwave oven (U.S. Pat. No. 4,241,114; 4,241,193; 4,273,886; 4,296,208; 4,305,796; 4,332,656). However, the problem of a difficulty in control of the physical property arose in which a chain extension reaction and foaming occur simultaneously. Further, in U.S. Pat. No. 4,319,000, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride (BTDA) and ethyl alcohol as esterifying agent were used to control the closed cell amount to the level below 95%. The polyimide foam was prepared by using two types of amines with a base dissociation constant of below 10.sup.-10, which were selected from the group consisting of 4,4-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, and 2,6-diaminopyridine. In order to obtain an polyimide foam with superior flexibility and homogeneous foaming structure, U.S. Pat. No. 4,369,261 used 3,3', 4,4'-benzophenonetetracarboxylic acid dianhydride, methylenedianiline (MDA), 2,6-diaminopyridine, and activated carbon or graphite with superior conductivity. In U.S. Pat. Nos. 4,647,597, 4,656,198 and 4,670,473, a polyimide foam was prepared by using 3,3', 4,4'-benzophenonetetracarboxylic acid dianhydride, methylenedianiline, and 2-methoxyethanol as plasticizer.
In U.S. Pat. Nos. 4,806,573, 4,824,874, and 4,830,883, 3,3', 4,4'-benzophenonetetracarboxylic acid dianhydride, organic polyisocyanurate, and furfuryl alcohol were used to prepare a polyimide precursor, which in turn was used as a reenforcing agent of a foam with an open foam structure.
In U.S. Pat. No. 4,952,611, a polyimide foam, with superior compression set property for a use in seat cushion, was prepared by using a mixture of 3,3', 4,4'-benzophenonetetracarboxylic acid dianhydride and 2-(vicynal-dicarboxycyclohexenyl)-succinic acid in addition to an amine selected from methylenedianiline or 2,6-diaminopyridine. diaminopyridine.
In order to obtain a cell size of 0.1 to 1.0 mm in a polyimide foam, in U.S. Pat. No. 4,978,692, an amine was selected form the group consisting of 3,3', 4,4'-benzophenonetetracarboxylic acid dianhydride, 4,4-bis[4-(3-aminophenoxy)phenyl]sulfide, and 4,4-bis[4-(3-aminophenoxy)phenyl]sulfone, 4,4-bis[4-(3-aminophenoxy)vinyl]benzophenon.
In U.S. Pat. No. 5,234,966, for preparing a polyimide foam of a target density, 3,3', 4,4'-benzophenonetetracarboxylic acid dianhydride, methylenedianiline, and 2,6-diaminopyridine was used, and a mixture of alcohol and water were used as esterifying agent.